The invention is particularly applicable to flicker photometers, which are used to measure the macular pigment in the human eye.
Macular pigment is the yellow pigment situated in the central portion of the human retina. The absorption spectrum for the pigment has a peak for light of a wavelength of 460 nm, and is zero for light of a wavelength of 540 nm, so that the pigment absorbs significant amounts of the shorter wavelength light whilst having little or no effect on light of the longer wavelength.
The highest concentrations of macular pigments are to be found in the region of the retina, the macula lutea (hereinafter referred to as the macula), which has a very high number density of cone receptors, and is coupled to a disproportionately large area of the visual cortex, giving that region a high degree of visual acuity. In fact, the macular pigment lies on a portion of the retina which corresponds to the center of the field of view of a subject (through that eye).
It has been proposed that the macular pigment protects the retina against harmful effects of short wavelength radiation, and that the pigment indeed provides protection against age related macular degeneration (AMD), a disease that leads to vision loss in the center of the visual field. Accordingly, much effort has been devoted to the noninvasive measurement of the density of macular pigment in the human eye.
To that end, a flicker photometer projects green and blue light (respectively of wavelengths of typically 540 nm and 460 nm) into a subject's eye so that the subject perceives a flickering stimulus in the center of his or her field of view. The subject can then adjust the intensity of one of the colors of light, typically the blue light. If an appropriate flicker frequency has been selected initially, the subject will be able to select an intensity of the blue light which is such that the subject perceives no, or a minimum amount of, flickering.
The intensity of the blue light is then determined, and this value can be used in calculating the macular pigment density. However, the selected intensity may be influenced by yellowing in the lens of the eye under examination, and this can vary from one subject to the next.
U.S. Pat. No. 5,936,724 and U.S. Pat. No. 6,017,122 show a flicker photometer in which the possible contribution of yellowing of the lens is eliminated by having the subject look at an offset mark and then repeating the intensity adjustment process until the perceived flicker of the stimulus, which is now in the subject's peripheral field of view, is minimized or eliminated.
However, this second type of measurement is found by some subjects to be a challenging task that often requires a period of training. One possible reason for this is that many subjects may find it difficult to concentrate on a stimulus in their peripheral field of view whilst resisting the urge to shift their view so that they are looking directly at the stimulus (which would then once again be on the macula).